Drilling fluid, often called “mud,” is, typically, a mixture of fluid and various additives which is pumped down through a hollow drill string (pipe, drill collar, bit, etc.) into a well being drilled and exits through ports or nozzles in the drill bit. The mud picks up drilled cuttings, debris, and other solids from the well and carries them upward away from the bit and out of the well in a space (annulus) between the well walls and the drill string. At the top of the well, the solids-laden mud is discharged. In many instances, it is fed to one or more shale shakers which have one or more screens for screening the material. A wide variety of vibrating screens and devices that use them (shale shakers) are known. The screens catch and remove solids from the mud as the mud passes through them so that the now screened mud can be reused and pumped back down the drill string. If drilled solids are not removed from the drilling mud being used during the drilling operation, recirculation of the drilled solids can create weight, viscosity, and gel problems in the mud, as well as increasing wear on mud pumps and other mechanical equipment used for drilling.
In drilling a wellbore, the circulation of drilling fluid to and then away from the drill bit can cease due to the porosity of the formation and/or due to fracturing of the formation through which the wellbore is being drilled. This is referred to as “lost circulation.” When lost circulation occurs, drilling fluid is pumped into the fractured formation, rather than being returned to the surface. Often circulation is lost at some specific depth where the formation is “weak”, and where a fracture extends horizontally away from the borehole. Expressions used to describe rocks that are susceptible to lost returns include terms like “vugular” limestone, “unconsolidated” sand, “rotten” shale, and the like.
A wide variety of “lost circulation materials” have been developed and pumped into wellbores to fill or seal off a porous formation or to fill or seal off a wellbore fracture so that a proper route for drilling fluid circulation is re-established. For purposes of classification, some lost circulation materials may generally be divided into fibers, flakes, granules, and mixtures.
It is often desirable to retain the lost circulation material in the drilling mud system during continuous circulation. Screening the drilling mud in the usual manner for removal of undesired particulate matter (e.g., drilled solids) can also result in the removal of the lost circulation material. Such screening can therefore require continuous introduction of new lost circulation material to the drilling mud downstream of the mud screening operation.
The addition of the lost circulation material into the drilling fluid compounds the separating problems because it, like the drilling fluid, is often cleaned and recirculated. The drilling fluid exits the well with solids that include: (1) valuable small sized particles such as clay minerals and weighting minerals, (2) valuable lost circulation material of a large size, and (3) undesirable drilled solids that span sizes from coarser than lost circulation material to sizes of the smallest of the valuable materials in the fluid. The function of the lost circulation material is to seal openings or gaps in an earth formation. Unfortunately, this lost circulation material, when pumped back to the surface of the well and passed through mud cleaning apparatus at the surface, can plug up separator components, e.g. fine screen cloth on shale shaker screens. One proposed solution to this separation problem is a conventional two step screening process as shown in U.S. Pat. No. 4,116,288 in which an exiting mixture of drilling fluid, lost circulation material and undesirable material is first subjected to a coarse screening to separate the lost circulation material from the drilling fluid and undesirable material which drops to a second finer screen there below to separate the drilling fluid from the undesirable material. The drilling fluid and lost circulation material are then reunited for recirculation into the well. This system is susceptible to height restrictions and fine screen problems and can allow undesirable solids or pieces of cuttings or debris of the same size as lost circulation material to be circulated back into a well. Often the moist, fibrous lost circulation material will also be coated with finer undesirable material which will not go through a first screen and which is therefore circulated back into a well.